1. Field of the Invention
The present invention generally relates to current supply circuits, and more particularly to a current supply circuit used as, for example, a bias current supply circuit of an amplifier circuit such as a bipolar integrated circuit amplifier.
It is known that, generally, bipolar transistors have a current transfer ratio which greatly depends on the junction area and the impurity density. When integrally forming a circuit formed of bipolar transistors on a chip, different current transfer ratios are obtained for different wafers due to various errors introduced in the production steps, and hence the circuits formed on the different wafers have different characteristics.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional constant-current circuit, which is made up of a constant-current source 101, an NPN transistor Q25 used to supply a constant current, and a load resistor R.sub.L. A constant voltage Vcc is applied to the constant-current source 101, which produces a constant current I based on the constant voltage Vcc. The current I is supplied to the base of the constant-current supply transistor Q25.
The emitter of the NPN transistor Q25 is grounded, and the collector thereof is coupled to the power supply line of the constant voltage Vcc via the load resistor R.sub.L. The collector current I.sub.C is expressed as follows: EQU I.sub.C =h.sub.FE I.sub.B ( 1)
where I.sub.B denotes the base current of the NPN transistor Q25, and h.sub.FE denotes the current transfer ratio thereof. Hence, the current I.sub.L flowing in the load resistor R.sub.L is written as follows: EQU I.sub.L =h.sub.FE I.sub.B ( 2).
In the above conventional constant-current circuit, the load current I.sub.L depends on the current transfer ratio h.sub.FE of the NPN transistor Q25. Hence, the load current I.sub.L varies in accordance with a variation in the current transfer ratio h.sub.FE.
Generally, the junction area and the impurity density slightly vary during the process of producing transistors and ICs. Hence, the transistors formed on different wafers or chips have different current transfer ratios. The different current transfer ratios vary the load transistors I.sub.L. As a result, circuit devices equipped with constant-current circuits as described above have different operation characteristics due to deviations in the load current I.sub.L.